VDAC, a voltage dependent anion-selective channel located in the outer mitochondrial membrane, has been studied after incorporation into planar lipid bilayers. This channel forms a large pore in a lipid bilayer membrane since 400 dalton ions can cross rather easily. The limiting size of the channel will be estimated by examining the permeability of larger molecules and ions. Electron microscopy will be done to examine both VDAC's structure and size. VDAC has been purified from rat liver mitochondria. It has an overall molecular weight of 120 kdal and, on SDS polyacrylamide gel electrophoresis, a single major band at about 34 kdal was observed. Crosslinking experiments will be used to determine if VDAC is a trimer or a tetramer. Modifying agents will be used to probe the channel's structure. Electrophysiological studies will be done to further characterize the various conductance states of VDAC and how the accessibility of these states varies with voltage and medium composition.